M.S. Tiwari, et al.
MolecularCatalysis488(2020)110918
an FID detector. The products were identified by GC–MS, synthetic
mixtures were prepared and used for quantification of the data.
3. Results and discussion
3.1. Catalyst characterization
3.1.1. X-ray diffraction spectroscopy
The X-ray diffraction patterns of all catalysts including K-10 clay, 20
% w/w TPA supported on K-10, 20 % w/w tin exchanged TPA sup-
ported K-10 with x = 0.5 to 1.5, and reused 20 % w/w Sn1TPA sup-
ported on K-10 were recorded and are shown in Fig. 1. X-ray diffraction
pattern of unsupported TPA and Sn1TPA is also shown as an insert in
Fig. 1. TPA and unsupported Sn1TPA showed crystalline nature, as can
be seen in the insert in Fig. 1. However, peak intensity decreased after
proton exchange with tin indicating the decrease in crystallinity. X-ray
diffraction pattern of K-10 showed peaks at 20° and 35° attributed to
110 and 105 facets of montmorillonite [29,37]. Apart from that all the
other peaks are due to impurities and other materials present in the K-
10. This includes quartz (peaks at 21.1°, 26.5°, 50.1°,59.8°), feldspar
(peak at 27.9°) and phengite (peaks at 36.6°,42.5°,45.6°), which con-
firms that K-10 is made of different phases as reported earlier [37–40].
X-ray diffraction patterns of TPA, and tin exchanged TPA supported on
K-10 were similar to those of the corresponding K-10. This could be
attributed to uniform distribution of Keggin anion in the non-crystalline
form because of the interaction with the surface of K-10 clay [25,29].
The X-ray diffraction pattern of reused and fresh 20 % w/w Sn1TPA
supported on K-10 catalysts were almost identical which confirmed the
stability of the catalyst after reactions.
Fig. 2. FT-IR spectra of catalysts (a) TPA, (b) K-10 clay, (c) TPA/K-10, (d) Sn0.5
-
TPA/K-10, (e) Sn1-TPA/K-10, (f) Sn1.5-TPA/K-10, and (g) reused Sn1-TPA/K-10.
methanol (5 ml) was added to the dried powder of tin loaded K-10 clay,
using same procedure as stated above to get nearly dry solid material.
The resultant material was then dried at 120 °C for 12 h in a tubular
furnace under flowing air, followed by calcination at 300 °C for 3 h to
get active 20 % w/w Snx-TPA/K-10 catalyst. The resultant 20 % w/w
Snx-TPA/K-10 catalyst was stored in an air tight bottle. 20 % w/w tin
exchanged TPA supported on bentonite clay catalyst was also prepared
by using same method as above. 20 % (w/w) TPA/K-10 catalyst was
prepared by directly following the second step of the preparation
method.
3.1.2. Fourier-transform infrared spectroscopy (FT-IR)
FT-IR spectra of all catalyst samples are shown in Fig. 2. It exhibits
characteristic bands at 790, 891, 987 and 1081 cm−1 related to the
Keggin structure of TPA, assignable respectively to the stretching vi-
bration of edge sharing WeOeW, corner sharing WeOeW, W]O
terminal and PeO bonds of the Keggin ion structure in pure TPA
[19,26]. FT-IR spectra of K-10, TPA/K-10 and tin exchanged TPA sup-
ported on K-10 were similar. The K-10 shows the intense band at
1010 cm−1 corresponding to SieO out of plane stretching while
shoulder at 935 cm−1 is due to SieO in-plane stretching vibration [37].
The FT-IR band in the region of 3410 cm−1 is due to asymmetric
stretching of the eOH group while the band in the region of
1600–1700 cm−1 corresponds to bending vibration of eHeOeH bonds
[40]. All catalyst shows the same bands as corresponding to K-10 be-
cause the bands related to Keggin ion of TPA were superimposed by K-
10 bands in the region of 700–1100 cm−1 as reported earlier [41,42].
FT-IR spectra of reused and fresh catalysts were also the same, and no
additional bands were observed. This indicates the supported Keggin
anion is stable under the reaction conditions.
2.3. Catalyst characterization
20 % w/w tin exchanged TPA supported on K-10 and bentonite, and
20 % w/w TPA supported on K-10 catalysts were characterized by X-ray
diffraction, framework IR, UV–vis and SEM analyses as well as surface
acidity measurement. The X-ray scattering measurements were made
with PANalytical X-Pert Pro MPD diffractometer with Ni filtered CuKα
radiation (1.5405 A°). The diffractograms were recorded with step size
of 0.016° from 5° to 80°. FT-IR spectra of samples were recorded on a
Perkin-Elmer Fourier Transform Infrared Spectrometer. Thin trans-
parent wafer like pellets were prepared by mixing 1 mg of catalyst with
100 mg of dried KBr and subsequent pressing.The UV–vis spectral
characterization was done using Shimadzu UV-1280 03540 spectro-
meter. SEM images were collected on FEI Quanta FEG 250 Scanning
Electron Microscope. The acidity of prepared catalysts was measured by
acid-base titration method as reported earlier [35,36]. The solid cata-
lysts were stirred in 25 ml of 0.1 M NaOH solution for 6 h, and then
titrated with 0.1 M HCl solution to get the acidity of the samples.
3.1.3. UV-VIS spectroscopy
UV-VIS spectra of K-10, TPA and TPA/K-10 are shown in Fig. 3. K-
10 showed no absorption bands indicating there is no absorbance of
light. TPA showed two absorption bands at 204 nm and 265 nm. These
bands could be attributed to the charge transfer from terminal oxygen
to tungsten and the charge transfer from bridge oxygen to metallic
tungsten respectively. These bands are characteristic bands of the TPA
polyanionic structure [29]. 20 % w/w Sn1TPA/K-10 showed the two
bands which confirmed that the Keggin ion is intact and supporting TPA
on K-10 does not affect the structure of TPA.
2.4. Reaction procedure
All the reactions were performed in 20 ml glass reactors equipped
with magnetic stirring Appropriate quantities of the reactants, FAL and
1-butanol were taken in a reactor and placed in oil bath at 110 °C and
agitated. The reaction mixture was stirred for 2 min at 1000 rpm and
then a zero hour sample was taken. Catalyst was added and stirring
started again, the reaction was carried out for 5 h and samples were
taken periodically for analysis. The reaction mixture was analysed by
using a HP-5 capillary column in an Agilent 7820A GC equipped with
3.1.4. SEM
The surface morphology of the catalysts was evaluated from SEM
analysis. The SEM images of 20 % w/w Sn1-TPA/K-10, fresh and reused
catalysts at different magnifications are shown in Fig. 4. These catalyst
3